The present invention concerns generally, and according to the first of its aspects, the cement industry. More specifically, the invention concerns, according to its first aspect, a manufacturing process of a cement from a first raw mix formed from a mixture consisting of the compounds CaCO3, Al2O3 and/or Al(OH)3, CaSO4, SiO2 and/or a product containing silica or silicates such as clay, all these compounds being present in anhydrous or hydrated form, individually or in combination.
Cements of this type, which are called sulfoaluminate cements, have been used since 1974 for constructions in China. These cements form a new class of construction materials which are different from the portland cements, which have been known for approximately 150 years, or aluminous cements invented by the Lafarge Company at the beginning of the 20th century.
The preparation of cement includes cooking of a base or raw mixture. The quality of the heat treatment for transformation of this raw mix into clinker will be determined to obtain cement with satisfactory mechanical properties during its use.
The kilns usually encountered in cement making are tubular and rotary. These kilns have been used for more than fifty years for the preparation of portland cements.
In a known way, a raw mix for obtaining sulfoaluminate cement is introduced at the first end of the center of an approximately horizontal tubular kiln in the form of flour, then is heated by the combustion gases derived from a burner placed at a second end of the center. The center of the tubular kiln is mounted so that it rotates around its axis of symmetry and is slightly inclined towards the burner, such that the rotation of the tube causes the rotation and the fall of the flour towards the burner at the second end of the center.
In proportion to its progression in the kiln, the flour is reheated and successively undergoes dehydration, decarbonation and clinkering.
However, because of the variability of the speed of movement of the flour in the center, this type of kiln does not make it possible to obtain a clinker of constant quality. In fact, it turns out that the flour remains stuck to the walls of the kiln and has a prolonged residence time. Likewise, the flour can go more rapidly from the first to the second end of the kiln without having been treated satisfactorily.
The use of raw mixes containing iron oxides to obtain ferroaluminate cements under conditions similar to those described above for the preparation of sulfoaluminate cements leads to worsening of the problems of sticking to the walls of the kiln. An increased crusting of the kiln is then observable, particularly in the clinkering zone where the iron oxides are in the liquid state.
Another solution can consist of using a kiln with a fixed center in which the raw mix is brought progressively to the clinkering temperature, then cooled and milled.
This technique has the drawback of being expensive because of the heat losses connected with the heating then the cooling of the kiln between each cycle. Furthermore, the operating method of this type of kiln is not suitable for production of large amounts of clinker. Since the mixture to be clinkered is immovable, heterogeneous zones appear in the clinker because of local variations in the temperature in the kiln.
In this context, the invention has the goal of proposing a process enabling the production of sulfoaluminate cement of constant quality under industrially acceptable conditions. These conditions include minimizing the consumption of energy which is a determining parameter in the cost of the cement.
To this end, the cement manufacturing process according to the invention moreover conforms to the generic definition that is given in the preamble above, is essentially characterized in that the mixture is treated up to the clinkering by movement in a kiln center, with a sheet of approximately constant thickness, at approximately constant speed, along a treatment path subjected to a positive gradient of temperature, and during a treatment time during which this mixture remains below its melting temperature, and in which the clinkered material forming a sulfoaluminate cement is cooled upon exit of the treatment path.
Such a process makes it possible to improve the respective drawbacks of the tubular kiln with a rotary center and kilns with a fixed center, by the movement at controlled speed of a predetermined amount of mixture to be clinkered in an atmosphere at controlled temperature and which can be regulated.
Furthermore, even because of the layout of the kiln, it is possible to vary the composition of the hot gases circulating in the center, and therefore, the selection of a locally oxidizing or reducing atmosphere; for example, by evacuating the flue gases produced in the clinkering zone in any of the parts of the kiln or even by varying the oxygen content during combustion.
In addition, the mixture can consist of Al2(SO4)3 in anhydrous or hydrated form. Al2(SO4)3 can represent up to 10 wt % of the mixture.
The composition to be treated by the process according to the invention can in addition consist of at least one mineral phase high in at least one iron oxidation product to obtain a second raw mix, said cement then forming ferroaluminate cement.
In addition, the mixture can consist of an oxide of transition metal chosen from Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Cd, La, Hf, Ta and W.
The transition metal oxide can represent up to 5-10 wt % of the mixture.
The process of the invention can consist of the preparation of the composition by steps of (i) mixing of each of its constituents with water to obtain a slurry, (ii) filtering said slurry to obtain a pasty residue, (iii) transforming said residue with a view to its introduction into the kiln.
According to the process of the invention, the first pasty raw mix can be transformed by mixing then extrusion, preferably in plates or in strips with approximately constant thickness.
The first pasty raw mix can also be extruded in the form of varied geometrical units, insofar as they make it possible to obtain a clinkered product of satisfactory quality at the end of the heat treatment because of the speed of diffusion of the heat to the interior of the section. Thus, cylindrical or ovoid rods or even hollow bricks can be appropriate.
During the use of a composition consisting of iron oxidation products, the second raw mix can be deposited before clinkering on an underlayer of the first raw mix.
Preferably, the movement of the composition in the center of the kiln is carried out on rollers.
The rollers present in a clinkering zone of the center of the kiln can be silicon carbide, possibly coated with silicon, or MgOxe2x80x94ZrO2.
Coatings of MgOxe2x80x94ZrO2 can be obtained by depositing under vacuum, according to a technique called CVD (Chemical Vapor Deposition).
The rollers present in a clinkering zone of the center of the kiln can be coated with a refractory stainless steel with a melting point higher than 1400xc2x0 C. These coatings of stainless steel can consist of sleeves mounted so that they each rotate freely around a roller. A preferred refractory stainless steel can be an alloy with a high content of nickel, for example, Inconel(copyright), Inconel 601(copyright) or even Inconel 706(copyright). It has been observed that during use between 1300 and 1400xc2x0 C., the sleeves mounted to freely rotate could possibly yield but presented good mechanical resistance to plating, wearing and corrosion. Therefore, they are particularly adapted for the manufacture of cement by the process of the invention.
Because of the particular composition of the raw mixes used in the invention, it is necessary for the clinkering to be carried out in an oxidizing atmosphere. This oxidizing atmosphere is obtained with the aid of gaseous oxygen in the clinkering zone of the raw mix.
According to the manufacturing process of the invention, the mixture undergoes, along the path of the treatment, successively, (I) possibly a drying and/or dehydration (II) decarbonation and (III) clinkering.
The clinkered mixture obtained by a process in conformance with the invention can be milled and mixed with limestone and/or gypsum and/or anhydrite and possibly heavy metals or oxidation compounds of heavy metals.
According to a second of these aspects, the invention concerns cement obtained according to a process in conformance with its first aspect.
According to a third of these aspects, the invention concerns a manufacturing installation of cement consisting of a tank and a mixer for preparing a first raw mix formed by a mixture consisting of the compounds CaCO3, Al2O3 and/or Al(OH)3, CaSO4, SiO2 and/or a product containing silica or silicates such as clay. All these compounds are present in anhydrous or hydrated form, individually or in combination.
The installation consists of a kiln and a kiln center, to treat the mixture up to clinkering by movement in said kiln center. The movement is carried out with a sheet of approximately constant thickness, at approximately constant speed, along a treatment path subjected to a positive temperature gradient, and during a treatment time during which this mixture remains below its melting temperature. The installation consists of equipment for rapid cooling of the clinkered mixture upon exit from the treatment path.
The rapid cooling of the clinker can be advantageously carried out by means of cooled gaseous nitrogen or even liquid nitrogen. The nitrogen used for cooling the clinker can originate advantageously from the distillation of the air in an installation making it possible to simultaneously produce on the one hand, liquid or gaseous oxygen and on the other hand, liquid or gaseous nitrogen. The oxygen produced is used during the clinkering step.
According to a preferred embodiment method, the installation of the invention can include (i) a first mixer associated with a tank for obtaining a slurry during the combination of the mixture with the water, (ii) a filter press for filtration of said slurry in order to obtain a pasty residue and (iii) an extruder for transforming said pasty residue, with a view to its introduction into the kiln.
This installation can in addition include a mixer for homogenization of the pasty residue derived from the filtration, and an extruder for its transformation into plates or strips with approximately constant thickness.
The center of the kiln can include rollers for movement of the mixture.
The installation according to the invention can use means for preparing a second cement raw mix including, in addition, at least one mineral phase high in at least one iron oxidation product. Such a second raw mix provides, after clinkering, a ferroaluminate cement.
The installation according to the invention in addition can consist of means for depositing the second raw mix on an underlayer of the first raw mix before clinkering.
The installation according to the invention can use a first grinding mill of clinkered mixture, a second mixer in which there is added to the clinkered mixture milled limestone and/or gypsum and/or milled anhydrite, and possibly heavy metals or oxidation compounds of milled heavy metals.
The invention uses several silos or hoppers, several conveyers, and several pipes for storage and transport of the products intended for the manufacture of sulfoaluminate cement.